1. Field of the Invention
The present invention relates to a magnetic recording medium and a production method thereof, and in particular, to a perpendicular magnetic recording medium, where reproduction noise is low and high density recording is possible, and a production method thereof.
2. Related Background Art
Increase in information recording capacity of magnetic disk units and the like is desired with sharp increase of information processing in recent years. In particular in hard disk drives, an information-recording amount per unit area is now increasing with an annual rate of 60% or more. The information-recording amount is desired to continue to increase, and miniaturization and a higher density are also desired for use as a portable recording device and the like.
In a magnetic recording medium for a hard disk drive used conventionally, a longitudinal magnetic recording method is adopted, and magnetization is recorded in parallel to a disk surface. In this longitudinal magnetic recording method, it is necessary to thin a magnetic recording layer to generate a magnetic field upward from the medium so as to suppress an anti-magnetic field in each magnetic domain and to detect a magnetization state, with high densification. Therefore, a volume of a magnetic particle becomes extremely small, resulting in the tendency of easily bringing a superparamagnetic effect. Thus, it may happen that energy stabilizing a magnetizing direction becomes lower than thermal energy, magnetization recorded changes with time, and finally record is erased. For this reason, in recent years, researches shifting to perpendicular magnetic recording methods where the film thickness of a recording layer can be made to be large have been active instead of the longitudinal magnetic recording method.
As media for perpendicular magnetic recordings, a monolayer type of medium having a single layer of magnetic recording layer, and a two-layer type of medium having a hard magnetic recording layer on a soft magnetic layer with high permeability that is a backing layer are proposed. In the case of the latter, a magnetic circuit is constituted, the magnetic circuit where a magnetic field, which is concentrated from a perpendicular magnetic head to the recording layer, is returned to the head through horizontally passing the soft magnetism layer. Although effects of increasing a recording magnetic field and enhancing record and reproduction are expected in this two-layer type of medium having the backing layer, it is also pointed out that there is a problem that a reversal of magnetization of the soft magnetic layer, and noise accompanying domain wall transfer, etc. are caused.
This will be described in detail below with using an explanatory diagram of a conventional perpendicular magnetic recording medium in FIGS. 2A and 2B. It is possible to use a glass substrate, an aluminum substrate, a carbon substrate, a plastic substrate, a Si substrate, etc. can be used as a substrate 21. In the case of the aluminum substrate, in order to secure hardness, as shown in FIGS. 2A and 2B, a NiP layer 22 is produced in many cases as a base layer by plating etc. As a backing layer 23, a NiFe alloy (permalloy) with high permeability etc. is used at the thickness of several μm to several tens μm. Generally, as a recording layer 24, a Co—Cr alloy is used. When being produced by sputtering, a recording layer 24 grows up in a state where a core portion 26 with much Co composition, and a shell portion 27 with comparatively much Cr composition around the core portion 26 are separated as shown in FIG. 2B. The core portion 26 has the hexagonal close-packed structure (hereinafter hcp structure) having an approximately cylindrical shape, and becomes hard magnetic to become a recording portion. The shell portion 27 becomes soft magnetic or non-magnetic due to much Cr composition, and also plays the role of weakening the interaction between adjacent core portions. In the core portion 26, since the c-axis faces in the direction perpendicularly to the substrate, magnetization turns in the direction perpendicularly to the substrate due to the action of crystal magnetic anisotropy. Ta, Pt, Rh, Pd, Ti, Nb, Ht, and the like are added besides Co—Cr in the above-described recording layer 24.
In addition, although this is not shown in FIGS. 2A and 2B, a base layer is formed between the recording layer 24 and backing layer 23 in order to enhance the crystallinity of the recording layer 24. Alternatively, in order to weaken a little the magnetic bond of the recording layer 24 and backing layer 23, a base layer such as an oxide layer is formed (refer to Japanese Patent Application Laid-open No. 7-73429).
It is common to thinly form a protection layer 25 on its surface, and carbon, carbide, nitride, and the like have been examined as materials.
Next, since the present invention uses anodic oxidized alumina having fine pores, an anodic oxide film and an alumite magnetic substance with the anodic oxidized film will be described in detail below with using FIGS. 3A and 3B. The term “anodic oxidized alumina” means a product through an anodic oxidization of aluminum.
When an aluminum substrate 31 is anodized in an acid electrolyte such as a sulfuric acid, oxalic acid, and phosphoric acid electrolyte, an anodic oxidized film 32 which is a porous anodic oxidized film as shown in FIG. 3A is formed (for example, refer to R. C. Furneaux, W. R. Rigby & A. P. Davidson, “NATURE”, vol. 337, p.147 (1989) or the like). The characteristic of this porous film is to have specific geometric structure that extremely fine cylindrical pores (alumina nanoholes 33) whose diameter 2r is several nm to several hundreds nm are arranged in parallel at intervals of several tens nm to several hundreds nm (2R)). This cylindrical pore has a high aspect ratio, and is also excellent in the uniformity of cross sectional diameters.
In addition, it is possible to control the structure of the porous film to some extent by changing the conditions of anodic oxidation. For example, it is known that it is possible to control to some extent pore intervals with an anodic oxidation voltage, the depth of pores by anodic oxidation time, and diameters of the pores by pore-widening treatment. Here, the pore-widening treatment is the etching of alumina, where wet etching with phosphoric acid is usually used.
In addition, in order to improve the perpendicularity, linearity, and independence of pores in the porous film, a method of two-step anodic oxidation is proposed, that is, the method which produces a porous film having pores exhibiting better perpendicularity, linearity, and independence by performing again anodic oxidation after once removing a porous film formed by performing anodic oxidation (“Japanese Journal of Applied Physics”, Vol. 35, Part 2, No. 1B, pp. L126-L129, Jan. 15, 1996). Here, this method uses a phenomenon that hollows of the surface of the aluminum substrate that are formed when the anodic oxide film formed by the first anodic oxidation is removed become starting points for forming the pores of the second anodic oxidation.
Furthermore, in order to improve the controllability of shape, intervals, and a pattern of pores in a porous film, a method of forming starting points of pores with using stamping, that is, a method is also proposed, the method which is for producing a porous film having pores showing the controllability of better shape, intervals, and a pattern by performing anodic oxidation after forming hollows, which are formed by pressing a substrate, having a plurality of projections on its surface, on the surface of an aluminum substrate, as starting points of pores (Japanese Patent Application Laid-Open No. 10-121292 or Masuda, “solid physics” 31, 493 (1996)). In addition, the technology for forming pores that are not honeycomb structure but concentric shape is reported by Okubo et al. in Japanese Patent Application Laid-Open No. 11-224422.
As shown in FIG. 3A, an insulating layer made of thick aluminum oxide is formed in each bottom of the above-described alumina nanoholes 33. Since electrodeposition into nanoholes is difficult if there is this insulating layer, a method of thinning the insulating layer in each bottom of the nanoholes, that Is, a method called an electric current recovery method is generally adopted. The electric current recovery method is a method of thinning the insulating layer in each bottom by gradually lowering an anodic oxidation voltage. However, since each thin insulating layer remains by this method, alternating current electrodeposition with nearly 10 to 50 V of high voltage becomes necessary for the electrodeposition into nanoholes. Since there is limitation in structure control of electrodeposition inclusion objects in the electrodeposition with such a high voltage, polycrystals are usually electrodeposited unevenly. Thus, even if Co is electrodeposited, it is impossible to evenly grow the c-axis, which is an axis where magnetization is easy, in the direction perpendicular to the substrate (refer to “IEEE Trans. Mag.” vol. 26, 1635 (1990), and the like). In addition, since the thickness and shape of insulating layers in bottoms of nanoholes are uneven, portions not electrodeposited are apt to arise as shown in FIG. 3B. Referring to FIG. 3B, an electrodeposited magnetic substance is indicated by 34, and an extended portion by 35.
There is much dispersion in the shape of particles, including MPt (M=Co, Fe, Ni), which has Co and L10 ordered structure, as a component in the above-described conventional recording layer formed by sputtering. It is said that, in a medium for perpendicular magnetic recording, the dispersion of coercivity normalized mainly with saturating magnetization and an average coercivity determines characteristics. Thus, the dispersion of particles in the size of MPt (M=Co, Fe, Ni) microcrystals that have Co and the L10 ordered structure is reflected in the dispersion of the coercivity of the particles as it is, and deteriorates the characteristics as a recording medium. Of course, the dispersion in orientations of crystallographic axes of the microcrystals is also a cause of the deterioration of the characteristics.
In addition, it is difficult to fill the pores with the above-described alumite magnetic substance, and it is not possible to control the crystal orientation of magnetic substances, and in particular, to control c-axis orientation. Furthermore, it is insufficient to control an amount of electrodeposition inside each pore.
An object of the present invention is to provide a perpendicular magnetic recording medium that has uniform crystal orientation to an anodic oxidation alumina layer, and in particular, the c-axis orientation of Co, a Co alloy, and MPt (M=Co, Fe, and Ni) having the L10 ordered structure.
In addition, another object of the present invention is to provide a perpendicular magnetic recording medium with good record and reproduction characteristics, in which record particles are shaped like pillars and variations in the shapes thereof are reduced.
Furthermore, still another object of the present invention is to provide an effective backing layer that enhances record and reproduction characteristics.
Moreover, further still another object of the present invention is to provide a method of easily manufacturing the above-described magnetic recording medium, and to provide a magnetic record and reproduction apparatus where the above-described magnetic recording medium is used.